The present disclosure is directed to a system for monitoring hydrocarbon leaks from an underground storage tank. It is also useful for detection of hydrocarbon invasion at, near or around leaking plumbing systems and the like for handling hydrocarbons. The present disclosure additionally describes a monitoring system which can detect leakage at and around fluid handling systems in petroleum refineries, fuel pump stations, and the like.
Consider, as one example, an underground storage tank which normally stores kerosene, gasoline, aviation fuel, or the like. In particular, consider those tanks which store large quantities of relatively light hydrocarbons. Light hydrocarbons are distinguished from heavier molecules such as those found in lubricants, greases, and the like. The lighter molecules tend to migrate more readily because they are much more volatile. In other words, there is a tremendous difference in the vapor pressure of selected products in contrast with grease and lubricating oils. Heavier oils and greases may need to be monitored, but they are involved with a different type of exposure. A typical storage tank filled with a more volatile mixture of hydrocarbons may well include some very light, highly migratory molecules such as benzene. The light molecules thus have greater permeation into the surrounding environment, and indeed, can be detected in the soil at, near and adjacent an underground storage tank or other fluid handling facility. Consider as an example a service station which dispenses products from underground storage tanks including storage tanks for gasoline, kerosene, aviation fuel, lubricants and the like. After installation, perhaps even a perfect installation, leakage may occur. It is possible that a storage tank will develop a leak. It is also possible that connections with valves, pipes, etc. will develop leaks at the flanges or interconnection points. It is possible also to periodically spill the liquid stored in the tank. Whatever the case, leakage is a recurrent problem in, next to and around a storage tank and particularly an underground storage tank. It is typical to place an underground storage tank and lines running to or from the tank on coarse sand or pea gravel. This protective layer is placed under the tank so that it may settle against the soil which conforms to the shape of the bottom of the tank. Moreover, the underground storage tank, if leaking, may leak at any location on the tank. It is possible to detect large or gross leaks by a loss of product. However, a leak can be quite small, sufficiently small that the leakage loss is simply too small to be detected. It should be noted that there are additional product loss possibilities such as evaporation; that is, the liquid in the tank may evaporate and the fumes escape to atmosphere. That can be sufficiently large that it will obscure the loss rate occasioned by an underground leak.
Many problems can arise with an underground storage tank and the associated leakage. The present disclosure is directed to a system whereby leakage can be detected. Detection is made difficult by virtue of the fact that the leakage will enter the soil unseen and may travel in several directions. Generally speaking, it permeates in all directions with impunity. The vapor phase of hydrocarbons will permeate in all directions. Liquid phase hydrocarbons will travel vertically, with limited wicking horizontally until they contact the saturated zone above the water table, at which point they will begin to travel in the direction of groundwater flow. A complicating factor is the height of the water table in the near vicinity. The water table typically will collect any contact hydrocarbons and carry them on the surface of the water table as a thin film. This, however, may change the direction and degree of permeation into the soil. For instance, the water table may redirect the leaking hydrocarbons in a particular direction or limit penetration in other directions.
The present disclosure is directed to a monitoring system. It contemplates the well drilling with a vertical slotted or perforated casing which is installed in the well. At least one, and usually several such wells are around a storage tank. For easy description, assume that a service station has a pair of adjacent underground storage tanks, each holding 5,000 gallons. Assume further that the sales at the service station will nearly empty both tanks in a week. Thereafter, a large transport truck delivers fuel to the service station to refill the partially or wholly empty tanks. Assume, for purposes of description, that the two large tanks are side by side. Proper monitoring suggests that any leak which is larger than about one liter per hour of the liquid product be detected with a confidence level of ninety percent or greater. To this end, assume that four vertical wells are drilled around the two tanks, conveniently being identified as north, east, south and west wells. The present disclosure sets out an apparatus to be installed in the wells for detection of the leakage. Assume also that there is the possibility of spillage at the surface where the tanks are filled. Again, if spillage can soak into the soil, that also needs to be detected. At the time the detection apparatus is installed, it is sensitive to the permeated hydrocarbons previously in the adjacent soil. The present disclosure sets forth a method for measuring the background level. Assume, for purposes of description, that the tanks have been in place for ten years. Assume further that they do not leak at all. Even so, a background level of a specified few parts per million (ppm) will be established for the four wells. Even when the tanks do not leak, the background levels will persist for a long period of time. Recent leakage must create a different data in contrast with the background level. Moreover, the background level must be determined as to particular types of hydrocarbons. Assume, for example, that the hydrocarbon background level is determined to be 10 ppm. Assume further that a tank is converted from storage of jet fuel to gasoline. The common molecules in the tank can penetrate by diffusion through the surrounding soil at different rates so that characteristic molecules must be determined by a testing technique described hereinafter. Accordingly, fresh or recent leakage can be sorted from background hydrocarbon data in or near the vicinity of the storage tank.
The present method and apparatus therefore sets forth a hydrocarbon spillage or leakage test procedure and equipment for implementing that test procedure. In the preferred embodiment, the hydrocarbon handling equipment, typically a storage tank, but including pipelines, valves, pump stations and the like, is encircled by three or four shallow wells. Wells are preferably placed in the porous backfill that surrounds the hydrocarbon handling equipment, and spaced in a manner that allocates the excavation into approximately equal volumes to minimize the distance that hydrocarbons must migrate to come into contact with a well. They are preferably parallel, drilled to a depth to intercept diffusing hydrocarbon molecules traveling through the soil. They form a circle around the facility. They preferably have a depth of about six feet as a minimum, perhaps thirty feet as a maximum, and do not need to be much deeper than the water table. The depth is tailored to the size of the equipment, usually the underground storage tanks. Background data is obtained as a preliminary step. The data is obtained by placing a perforated casing in each hole and suspending on a flexible cable or line a small buoyant cartridge of absorbent material. The preferred material is divinylbenzene (DVB) which is preferably formed into particles sized in a range of screens. The DVB is an hydrophobic absorbent for hydrocarbons diffusing through the soil. Particles are enclosed in a perforated cylindrical container or canister. They preferably absorb and hold a specified quantity of hydrocarbons. The wells are sealed after placing such a cartridge in the wells, and the cartridges are removed on a fixed schedule such as once per month. The cartridges are tested by forcing air through them at a test facility, the air being directed through a flame so that flame detection indicates the presence of combustible hydrocarbons. Calibration of the flame output provides an indication of concentration, and contrasted with background data, an indication is obtained indicative of a spill or leak.